Treatment of oil sands tailings with lime at elevated pH levels

ABSTRACT

Methods and systems for treating oil sands tailings streams at an elevated pH using lime are disclosed herein. In some embodiments, the method comprises providing a tailings stream including 10-55% solids by total weight, increasing the pH of the tailings stream by combining the tailings stream with lime to produce a lime-tailings mixture having a pH of at least 11.0, and dewatering the lime-tailings mixture to produce a first stream having 10% or less solids by total weight and a second stream having 50% or more solids by total weight. The first stream can correspond to a release water stream, and the second stream can correspond to a cake. The lime slurry can include about 10% lime by total weight, and can comprise lime hydrate, quicklime, or a combination thereof. Dewatering the lime-tailings mixture can include routing the lime-tailings mixture to a centrifuge unit and/or a pressure or vacuum filtration unit.

TECHNICAL FIELD

This application relates to systems and methods for dewatering oil sandstailings. In particular, oil sands tailings are mixed with a limeadditive to promote dewatering of the oil sands tailings.

BACKGROUND

The extraction of bitumen from oil sands has been traditionallyperformed using the Clark Hot Water Extraction (CHWE) process. Atailings slurry, defined as whole tailings, is produced as a byproductof the CHWE process, and can include water, sand, clay, and residualbitumen particles that are suspended in the extraction process water.Coarse sand particles (e.g., >44 μm) can be easily removed fromwhole-tailings, but removal of finer particles (fines) can be moreproblematic. A portion of the remaining fines, water, and residualbitumen form a slurry that is about 10-15% solids by mass, which after anumber of years can settle to be about 20-30% solids by mass. Thisslurry is referred to as fluid fine tailings (FFT) and/or mature finetailings (MFT), and can remain for decades in a fluid state withoutfurther aggregation or settling. Slow consolidation, limited solidsstrength, and poor water quality of the FFT/MFT limits options forreclamation and has resulted in the formation of large tailings ponds.

A number of different technologies have been tried to improve thereclamation of FFT/MFT. Some of these technologies includewhole-tailings treatment, non-segregating tailings (NST) production,composite tailings (CT) production, tailings reduction operations (TRO),atmospheric drying, or treatment with polymers. These methods, however,have worked with only limited success, as there currently exists over abillion cubic meters of FFT/MFT in tailings ponds. As such, there is aneed for an improved method and process to treat oil sands tailings toprovide an effective reclamation option.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of an oil sands tailings dewateringsystem, configured in accordance with an embodiment of the presenttechnology.

FIG. 2 is a block diagram of a method of dewatering an oil sandstailings stream, configured in accordance with an embodiment of thepresent technology.

FIG. 3 is a flowchart of a method of dewatering an oil sands tailingsstream, configured in accordance with another embodiment of the presenttechnology.

FIG. 4 is a graph that shows the effect of lime concentration on thesolids content of a lime-tailings mixture.

FIG. 5 is a graph that shows the effect of lime concentration onparticle size of solids in a lime-tailings mixture.

FIG. 6 is a bar graph that shows the measured pH levels of alime-tailings mixture as a function of the concentration of lime in alime-tailings mixture.

FIG. 7 is a bar graph that shows the relationship of the yield stress ofsolids in a lime-tailings mixture to both the concentration of lime andthe dewatering time.

FIG. 8 is an image that shows the relationship of the lime-tailingsmixture and the concentration of lime in a lime-tailings mixture.

FIG. 9 is a bar graph that shows the relationship between the solidscontent of the separated cake material and concentration of lime.

FIG. 10 is a bar graph that shows the relationship between theconcentration of lime in a lime tailings mixture and the rate at whichrelease water is separated from an FFT sample using pressure filtration.

FIG. 11 is a graph that shows the effect of pH on the number of freecalcium cations in a lime-tailings mixture.

FIG. 12 is a graph that shows the relationship between the pH of alime-tailings mixture and the concentration of lime in a lime-tailingsmixture.

DETAILED DESCRIPTION

A method and system of dewatering oil sands tailings using a limeadditive is described in detail herein in accordance with embodiments ofthe present technology. Numerous specific details are set forth in thefollowing description and figures to provide a thorough and enablingdescription of embodiments of the technology. One skilled in therelevant art, however, will recognize that the technology can bepracticed without one or more of the specific details. In otherinstances, well-known structures or operations are not shown or are notdescribed in detail to avoid obscuring aspects of the technology. Ingeneral, alternatives and alternate embodiments described herein aresubstantially similar to the previously described embodiments, andcommon elements are identified by the same reference numbers.

FIG. 1 is a schematic flow diagram of an oil sands tailings dewateringsystem 100 (“system 100”). The system 100 includes a tailings holdingreservoir 102 (e.g., a pond, diked area, tank, etc.) including oil sandstailings, and a lime holding reservoir 104 (e.g., a tank) including alime slurry (e.g., a lime additive). The oil sands tailings can includewhole-tailings (WT), thin fluid tailings (TFT), fluid fine tailings(FFT) and/or mature fine tailings (MFT) (referred to collectively as“tailings”), and can have a solids content of about 10-45 wt. %. Thelime slurry includes a liquid (e.g., water) and a lime additive. Thelime additive is less than 15 wt. % of the lime slurry, and is composedof inorganic materials that provide divalent (e.g., calcium) forcoagulation of tailings. The tailings stream in the tailings holdingreservoir 102 is slightly alkaline, having a pH level of about 7.5-8.5,and the lime slurry in the lime holding reservoir is alkaline, having apH level greater than or equal to about 12.0. The tailings and limeslurry are each directed to a mixing apparatus 106 (e.g., a mixer) wherethey are combined to produce a lime-tailings mixture having a pH of atleast 11.0. In some embodiments, the pH of the lime-tailings mixturevaries from about 11.0-13.0, about 11.5-12.5. The lime-tailings mixtureis then moved to a dewatering device 108 to promote dewatering of thelime-tailings mixture. As explained in further detail below, thedewatering device 108 can include a centrifuge, and/or a pressure orvacuum filtration system that separates the lime-tailings mixture into afirst stream (e.g., a centrate or a filtrate) substantially comprisingrelease water, and a second stream substantially comprising solids(e.g., a “cake”). The first stream including the release water centrateor filtrate can include a solids content less than about 5 wt. %. Thefirst stream can be directed to a pond and/or recycled back into thesystem 100 to be combined with the tailings stream prior to being mixedwith the lime slurry. The recycled portion of the release water caninclude soluble calcium cations previously injected as part of the limeslurry, and thus can decrease the amount of additional lime in the limeslurry needing to be injected to the mixing apparatus 106. The secondstream can include a solids content greater than about 55 wt. %. Forexample, the second stream can include a solids content from about 55-75wt. %. The second stream can be collected and transported using a truck,belt, and/or other conveying systems to an external site (e.g., atemporary storage or reclamation area).

The tailings stored in the tailings holding reservoir 102 can includewater, sand, silt, clay, and residual bitumen particles that aresuspended in the extraction water. The tailings can be obtained fromtailings ponds or the oil sands extraction process directly. Aspreviously described, the tailings may be stored in a tailings pond andinclude a settled solids content of about 10-45 wt. %. Morespecifically, the tailings can include a mineral solids content fromabout 30-40%, a bitumen content from about 1-3%, a clay content fromabout 60-80%, and a pH from about 7.5-8.5. Prior to being held in theholding reservoir 102, the tailings may undergo upstream processing,such as cyclone separation, screen filtering, thickening and/ordilution. The tailings stream entering the mixing apparatus 106, afterpotentially being combined with recycled water 114, is preferably above20 wt. % solids.

The lime slurry holding reservoir 104 can include an agitated tank. Thelime slurry additive stored in the lime holding reservoir 104 caninclude calcium-containing inorganic materials that provide divalentcalcium cations for coagulation of tailings. As such, the lime slurrycomprises a lime product including hydrated lime (e.g., calciumhydroxide (Ca(OH)₂)), quicklime (e.g., calcium oxide (CaO)), or enhancedhydrated lime. The enhanced hydrated lime includes particles with BETsurface areas exceeding 30 m²/g. In some embodiments, the lime slurrycan include dolomitic lime (e.g., lime including at least 25%magnesium), other lime-containing materials, or a combination ofquicklime, limestone, hydrated lime, enhanced hydrated lime, dolomiticlime, and/or other lime-containing materials. In the lime manufacturingprocess, limestone (e.g., calcium carbonate (CaCO₃)), is crushed to ½″to 2″ particles used as kiln feed. The kiln feed is then calcined, whichconverts the limestone particles into calcium oxide, which is sometimesreferred to as quicklime. Introducing water to the quicklime leads tothe formation of fine particles of hydrated lime, which is oftenreferred to using the generic term “lime.” In some embodiments, the limeadditive is in a solidified form. For example, the lime additive may bea powder that is formed by crushing or pulverizing larger pieces of alime-based solid (i.e., quicklime, limestone, hydrated lime, enhancedhydrated lime, dolomitic lime, or any combination thereof). The limeadditive is combined with a liquid, such as water, to form the limeslurry. For example, the lime slurry can compromise less than about 15wt. % lime, less than about 10 wt. % lime, or less than about 5 wt. %lime.

The tailings and the lime slurry additive are combined in the mixer 106to produce the lime-tailings mixture. The mixer 106 can include aholding tank and means to agitate the lime-tailings mixture, such asrotating blades. In some embodiments, the mixer 106 can include a staticmixer, a dynamic mixer, or a T mixer. The residence time in the mixer108 for particles of the lime-tailings mixture can vary from at leastabout 90 seconds, to at least about 5 minutes, to at least about 10minutes, or to at least about 20 minutes. In general, the mixer 106mixes the lime-tailings mixture to ensure the lime-tailings mixtureleaving the mixer 106 is well mixed and has a desired pH. The pH of thelime-tailings mixture determines the reactions occurring within themixer (e.g., cation exchange and/or pozzolanic reactions), as describedin more detail below. The pH of the lime-tailings mixture at the outletof the mixer can be measured and used to increase or decrease the pH ofthe lime-tailings mixture by (a) increasing or decreasing the feed rateof the lime slurry, and/or (b) increasing or decreasing the residencetime of the particles of the lime-tailings mixture. As will be discussedin greater detail below, the dewatering rate of the lime-tailingsmixture is affected by the pH level of the lime-tailings mixture, andincreasing the pH level of the lime-tailings mixture by, for example,increasing the amount of lime slurry can result in an increaseddewatering rate of the lime-tailings mixture. In some embodiments, otheradditives (e.g., polymers, defoamers), can also be included in thelime-tailings mixture to facilitate the dewatering process. In theseembodiments, the pH of the lime-tailings mixture is 11.0 or greater.

The lime-tailings mixture is directed, via gravity and/or a pump, fromthe mixer 106 to the dewatering device 108. As previously mentioned, thedewatering device 108 can include a centrifuge, a filtration systemand/or other similar systems that can provide a physical force on thelime-tailings mixture to promote dewatering. The centrifuge separatesthe lime-tailings mixture into release water (e.g., a centrate) andcake. In some embodiments, the centrifuge includes a scrollcentrifugation unit. In other embodiments, the centrifuge may be a solidbowl decanter centrifuge, screen bowl centrifuge, conical solid bowlcentrifuge, cylindrical solid bowl centrifuge, a conical-cylindricalsolid bowl centrifuge, or other centrifuges used in the relevant art.The filtration system can also separate the lime-tailings mixture intorelease water and cake, and can include a vacuum filtration systemand/or pressure filtration system to separate the release water from thecake. In general, the dewatering device 108 may be another type offiltering apparatus known in the relevant art that utilizes any desiredfiltration process. In some embodiments, the filtration system caninclude a Whatman 50, 2.7 micron filter and can subject thelime-tailings mixture to about 100 psig of air pressure.

The lime-tailings mixture may be transferred to the centrifuge or filterimmediately after the mixing process has completed or may be retainedfor a period of time to allow the dewatering process enough time toproceed. In some embodiments, the lime-tailings mixture may be retainedfor one hour or less. In other embodiments, the lime-tailings mixturemay be retained for more than one hour (e.g., one day, one week, onemonth, etc.). In general, the lime-tailings mixture may be retained forany desired amount of time to ensure that the lime-tailings mixture hasbeen modified enough for the centrifuge and/or filter to separate asufficient amount of water from the solids in the lime-tailings mixture.

The dewatering device 108 has a first outlet used to transfer theseparated release water, and a second outlet that is used to transferthe separated cake. The separated cake is a soft solid that is composedof the particulate matter found in the tailings, such as sand, silt,clay, and residual bitumen. The lime additive particles and someresidual water does not get removed during the dewatering process. Aspreviously mentioned, the cake can include at least 55 wt. % solids. Inother embodiments, the cake can include at least about 60% wt. % solids,at least about 65% wt. % solids, at least about 70% wt. % solids, atleast about 80% wt. % solids, at least about 85% wt. % solids, or atleast about 90% wt. % solids. In general, the cake may include a greaterpercentage of solids by weight than the percentage of liquids by weight.

The separated release water includes water found in the oil sandstailings and any water that may be found in the lime slurry. Theseparated release water may also contain some solid particulate matterthat is not separated from the water during the dewatering process, suchas sand, silt, clay, and residual bitumen, and the lime additive. In oneembodiment, the release water includes less than about 5 wt. % solids.In other embodiments, the release water can include less than about 10wt. % solids, less than about 4 wt. % solids, or less than 1 wt. %solids. In general, the release water includes a significantly greaterpercentage of water by weight than the percentage of solids by weight.

The release water may be directed to a number of different applications.For example, the release water may be (a) recycled back to the tailingstreatment process, or (b) used to pretreat extraction process water. Therelease water can be treated with carbon dioxide to reduce the pH andamount of soluble calcium cations present therein. This can be done vianatural absorption of bicarbonates (e.g., by carbon dioxide present inthe atmosphere), or by actively injecting carbon dioxide. In someembodiments wherein the release water is recycled back to the tailingstreatment process, at least a portion of the release water is recycledand added into the tailings holding reservoir 102 or the tailings streambeing transferred to the mixer 106. The recycled release water mixeswith the tailings prior to being combined with the lime slurry. Addingthe recycle water 114 to the tailings stream prior to the mixer 106increases the pH level of the tailings because the recycle waterincludes soluble calcium cations that were not removed during thedewatering process, and is thus alkaline. As will be discussed ingreater detail below, the recycle water 114 includes calcium ions thatwill readily react with carbonates present in the tailings stream toform insoluble compounds that precipitate out of solution and separatefrom the suspended tailings. Using recycle water 114 to reduce theamount of bicarbonates in the tailings reduces the amount of the limeslurry needed for enhanced dewatering to occur, which in turn reducesthe cost of the overall dewatering process. In some embodiments, usingrecycle water 114 to increase the pH level of the oil sands tailings canbe omitted and the oil sands tailings dewatering system 100 may not useany portion of the release water during the dewatering process.

FIG. 2 is a block diagram of a method 200 of dewatering a tailingsstream, configured in accordance with an embodiment of the presenttechnology. Process portion 202 includes providing a tailings stream toa dewatering system (e.g., the system 100). The tailings stream can havea composition similar to the tailings stream previously described. Thetailings stream may operate as a steady state system having a constantfeed or as a batch stream in which tailings are provided to the systemat regular intervals.

Process portion 204 includes adding a lime additive, such as quicklime,limestone, hydrated lime, or dolomitic lime to the tailings stream toform a lime-tailings mixture. Adding the lime additive to the tailingsstream increases the pH of the tailings stream, which increases the rateof the dewatering process and the solid wt. % of the cake. Specifically,the calcium hydroxide ions increase the pH of the tailings stream andprovide divalent cations that modify and affect the stability of fineclay soils in the tailings. Kaolinite (Al₂Si₂O₅(OH)₄) is a type of claytypically found in the oil sands tailings. As the pH increases above11.5, the calcium cations from lime are more soluble due to thedepletion of bicarbonates in process water. Above this pH, these solublecalcium cations can replace cations such as sodium and potassium on thesurface of the clay. As pH levels increase above 12.0, a chemicalmodification of the clay's surface occurs by pozzolanic reactions. Inpozzolanic reactions, soluble calcium cations from the lime react withsilicic acid (Si(OH)₄) and aluminate (Al(OH)₄ ⁻) functional groups fromthe Kaolinite to form calcium silicate hydrate (CaH₂SiO₄.2H₂O) andvarious aluminum hydrates, such as calcium aluminate hydrate. Afterbeing chemically modified, the fine Kaolinite particles grow in size,decrease their water layer and can be separated from the water using acentrifuge or filter, as previously described.

The cation exchange and pozzolanic reactions between Kaolinite and limedo not readily occur if the pH level of the lime-tailings mixture isbelow about 11.5 because the lime-tailings mixture lacks soluble Ca²⁺cations to react with the Kaolinite. Calcium cations from lime additivesare consumed by reactions with bicarbonates at lower pH. This isdifferent than other calcium cations like gypsum and calcium chloridethat have partially soluble calcium cations at lower pH. For example,when sodium bicarbonate is exposed to calcium hydroxide, calcium cationsbond with carbonate ions and sodium bicarbonate is converted to sodiumcarbonate (Na₂CO₃), as seen in Reaction 1:Ca(OH)₂+2NaHCO₃→CaCO₃+Na₂CO₃+2H₂O  (1)

The calcium hydroxide will also readily react with the sodium carbonateformed during Reaction 1 to form additional calcium carbonate and sodiumhydroxide (NaOH), as seen in Reaction 2:Ca(OH)₂+2Na₂CO₃→CaCO₃+2NaOH  (2)

The calcium carbonate formed during Reactions 1 and 2 will precipitateout of solution into solid particulate matter. Potassium bicarbonatewill undergo similar reactions with calcium hydroxide. In addition tothe bicarbonates found in the oil sands tailings, atmospheric carbondioxide (CO₂) will dissolve in water that has an alkaline pH level toform carbonic acid (H₂CO₃), which reacts with calcium hydroxide to formcalcium carbonate and water, as shown in Reactions 3 and 4:CO₂+H₂O→H₂CO₃  (3)Ca(OH)₂+H₂CO₃→CaCO₃+2H₂O  (4)

While Reactions 3 and 4 reduce the amount of calcium cations availablefor cation exchange and pozzolanic reactions to occur, the concentrationof carbon dioxide in the atmosphere is relatively low, and Reactions 3and 4 require longer periods of time to have an effect on theconcentration of free calcium cations in the lime-tailings mixture underatmospheric conditions. Reactions 1 and 2, on the other hand, arelimited only by the availability of carbonate ions in the lime-tailingsmixture and occur significantly more readily than cation exchange orpozzolanic reactions, which means that there are very few free calciumcations available to react with the Kaolinite. However, as the amount oflime additive added to the lime-tailings mixture increases, the pH levelof the mixture will eventually approach about 11.0 and the concentrationof carbonate ions in the mixture will approach zero. At this point, thenumber of free and soluble calcium cations in the water will increase.

As the pH level of the mixture increases to 11.0 or higher, settling ofthe solid particulate matter in the lime-tailings mixture alsoincreases. However, the dewatering rate of the mixture is still limitedat that pH. Only once the pH level of the mixture reaches a pH levelgreater than 12.0, and preferably about 12.3, will pozzolanic reactionsbetween the dissolved calcium cations and the clay particulate matteroccur which results in improved dewatering. As such, more lime additivemay be required to increase the pH level of the mixture above 12, sothat pozzolanic reactions between the soluble calcium and claycomponents can begin.

In systems where the oil sands tailings stream is provided as acontinuous flow of oil sands tailings, the lime additive may be acontinuous flow of lime additive that is added and mixed into the oilsands stream. In systems where the oil sands tailings stream is providedas batches of oil sands tailings, the lime additive may also be addedand mixed into the oil sands in individual batches in tandem with theoil sands tailings batches.

After the lime-tailings mixture has been thoroughly mixed, the methodproceeds to process portion 206, where the lime-tailings stream isdewatered by separating the solid material from the liquid components inthe lime-tailings stream. As previously described, the lime-tailingsstream dewatering process using a centrifuge and/or filter to forciblyseparate the solid material in the lime-tailings stream from the liquidcomponents. Specifically, the centrifuge and/or filtration systemprovide a driving force that promotes dewatering via the cation exchangeand pozzolanic reactions previously described. In other embodiments, thelime-tailings stream is dewatered using a tailings pond to allow thelime-tailings stream to dewater over time without the use of additionalmachinery.

After dewatering, the method proceeds to process portion 208 in whichthe dewatering system produces a cake with a solids content of at least55 wt. %. The solids in the cake are typically sand, silt, clay,residual bitumen, and the lime additive, along with any other solidparticulate matter that is present in either the lime-tailings stream orlime slurry additive. The balance of the cake is composed primarily ofwater that was introduced in either the tailings or lime slurry streams.As previously described, the dewatering system also produces a releasewater stream that is formed from the tailings water from which thesolids are separated. Converting the solid material found in the oilssands tailings stream into a stream of cake that is at least 55% solidsby weight enables significantly easier storage, transport, disposaland/or reclamation of the solid than when the solid material was trappedin suspension in the oil sands tailings stream.

FIG. 3 is a schematic flow diagram of a method 300 of dewatering an oilsands tailings stream. Specifically, the method 300 uses pH as a processcontrol parameter to promote dewatering. Characteristics of oil sandsore, such as the sand to fine ratio, can change significantly. It hasbeen difficult to control the dosage of coagulants and flocculants inoil sands mining operations because measurement of these changes in thetailings treatment process is difficult. As seen in FIG. 2, measurementof pH can be used as an effective method to understand the proper doseof lime that is required for dewatering. At step 302, a lime additive isadded to oil sands tailings, as previously described. At step 304, thelime additive and the oil sands tailings are mixed together to form auniform mixture. If desired, step 304 may be performed using a mixer toensure that the mixture is uniform. At step 306, the pH level of thelime-tailings mixture is measured and at step 308 the measured pH levelis compared to a predetermined threshold pH level to see if the measuredpH level is greater than or equal to the predetermined threshold pHlevel. As previously mentioned, the dewatering rate of the oil sandstailings is at least partially dependent on the pH level of the oilsands tailings. The predetermined threshold pH level is greater thanabout 11.0. In some embodiments, the predetermined threshold pH level isabout 11.5-13.0, or about 12.2. If the measured pH level of thelime-tailings mixture is not greater than or equal to the predeterminedthreshold pH level (e.g., it is less than the predetermined threshold pHlevel), the method reverts back to step 302 to add additional lime tothe lime-tailings mixture to further increase the pH level. This cyclecontinues until the pH level of the lime-tailings mixture is greaterthan the predetermined threshold pH level. When the measured pH level ofthe lime-tailings mixture is greater than or equal to the predeterminedthreshold pH level, the method proceeds to step 310. At step 310, thelime-tailings mixture is transferred to a centrifuge (or filter), whichcentrifuges (or filters) the lime-tailings mixture to separate thesolids from the water and produces release water and cake as separateoutputs.

After the centrifuging (or filtering), the method proceeds to eitherstep 312 and step 316. At step 312, the system measures the amount ofsolids remaining in the release water. At step 316, the systemdetermines the amount of solids in the cake. If the measured amount ofsolids remaining in the release water is too high or if the measuredamount of solids in the cake is too low, the system may adjust theoperating parameters of the system to ensure that the desired level ofseparation occurring. In one embodiment, the system determines that ifthe amount of solids remaining in the release water output from thecentrifuge (or filter) is at or below a given threshold. If the amountof solids in the release water is at or below the given threshold, themethod proceeds to step 314. If not, the system may adjust the operatingparameters of the system by increasing the amount of time that themixture rests for before being transferred to the centrifuge/filter, maymake a lime slurry dose adjustment and/or may adjust the operatingparameters of the centrifuge/filter. In another embodiment, the systemdetermines if the amount of solids remaining in the cake that is outputfrom the centrifuge (or filter) is at or above a given threshold. If theamount of solids in the cake is at or above the given threshold, themethod proceeds to step 318. If not, the system may adjust the operatingparameters of the system by increasing the amount of time that themixture rests for before being transferred to the centrifuge/filter,make a lime slurry dose adjustment and/or may adjust the operatingparameters of the centrifuge/filter. By constantly adjusting theoperating parameters of the system when the release water or cake outputby the centrifuge/filter do not meet the desired threshold requirements,the system is able to ensure that later batches of lime-tailings mixture(or later portions of the same lime-tailings mixture stream that haveyet to be processed for systems that operate continuously) will meet thedesired threshold requirements.

At step 314, the release water is either recycled, by being added to theoil sands tailings stream prior to the introduction of the lime additivein step 302, as previously discussed, or is deposited into a pond forstorage. In an alternative embodiment, the release water is treated withcarbon dioxide injection prior to being deposited into a pond. As willbe discussed in greater detail below, carbon dioxide readily reacts withcalcium cations and exposing the release water to carbon dioxide can beused to remove excess calcium from the release water, making the releasewater more suitable for release back into the environment and/or theextraction process. At step 318, the cake is removed from the system viaa truck, belt, or other transportation means and is transferred to adeposit site.

EXAMPLES

FIGS. 4-12 show results of examples and tests that corroborate theembodiments described above. Each of the FIGS. 4-12 are described indetail below and include tests run at different dosages (e.g., ppm) ofcalcium oxide or calcium hydroxide.

FIG. 4 is a graph that shows the effect of lime concentration in thelime-tailings mixture on the solids content of an FFT tailing sample.The slope of each line in the graph is indicative of the dewatering rateof that specific sample as it shows the change in the solids content ofthe mixture over time. Table 1 includes pH levels associated with thedifferent dosages described below for FIG. 4.

TABLE 1 pH levels associated with lime dosages in FIG. 4 Lime DosageMineral Solids Content % 0 8.24 1,000 9.37 2,000 10.76 3,000 11.91 4,00012.22 5,000 12.34 10,000 12.45

As shown in FIG. 4, a tailings stream that contains no lime additive(e.g., line 401) will have a very slow dewatering rate and will havelowest solids content at each time interval. This is in part because thehigh sodium levels on the surface of the Kaolinite clay hold watertightly which promotes dispersion and prevents settling. A lime-tailingsmixture that has a lime concentration of 1,000 parts per million (ppm)(e.g., line 402) has a slightly higher solids content at each timeinterval than the mixture that has zero lime additive but the differenceis relatively minor. As more lime is added to the lime-tailings mixture,the dewatering rate, and therefore the solids content at each timeinterval, increases. The lime-tailings mixture with a lime concentrationof 2,000 ppm (e.g., line 403) has a noticeably steeper slope than line402, which is indicative of a higher dewatering rate. As seen in Table1, at a dose of 2000 ppm the pH was below the 11.5 threshold for cationexchange to occur. Similarly, the lime-tailings mixture with a limeconcentration of 3,000 ppm (e.g., line 404) is even steeper. The pH ofthis 3,000 ppm dosage was 11.91 which indicated that cation exchange wasbeginning. Lines 405, 406, and 407, which represent mixtures with limeconcentrations of 4,000 ppm, 5,000 ppm, and 10,000 ppm, respectively,are even steeper still. The pH of these dosages were in the range ofpozzolanic reactions from 12.2 to 12.45. As shown by the graph in FIG.4, as the concentration of lime additive in the lime-tailings mixtureincreases, the dewatering rate continues to increase and the solidscontent at each time interval increases.

FIG. 5 is a graph showing the relationship between the concentration oflime additive in the lime-tailings mixture and the particle size of thesolids in the lime-tailings mixture. The horizontal axis of the graphrepresents the size of a given particle on a logarithmic scale and thevertical axis shows the cumulative percentage of the particles at eachparticle size for a given sample. In other words, each point on a givenline represents the percentage of particles in a given sample that havea particle size equal to or less than that particular particle size ofthe graph. For example, line 501 represents the particle sizedistribution of particles in a sample of the lime-tailings mixture thathas a lime additive concentration of 0 ppm. For the 0 ppm sample, 19.5%of all particles in the sample have a particle size that is less than orequal to 1 micron (μm), 42% have a particle size less than or equal to 2μm, 72% have a particle size less than or equal to 5 μm, and 88% have aparticle size less than or equal to 10 μm. For the 6000 ppm sample(e.g., line 502), 4% of all particles in the sample have a particle sizethat is less than or equal to 1 μm, 9% have a particle size less than orequal to 2 μm, 39% have a particle size less than or equal to 10 μm, and84% have a particle size less than or equal to 40 μm. In general,samples that have a greater percentage of particles with higher particlesizes are represented by lines that are further to the right in thegraph in FIG. 5.

Furthermore, the steepness of the slope of a line at a given particlesize is indicative of the number of particles in the sample that havethat particle size, such that the number of particles for a point on aline that has a steep slope is greater than the number of particles fora different point on the line that has a shallow slope. As can be seenby FIG. 5, the lime-tailings mixture samples that have higherconcentrations of lime additive tend to have steep slopes at largerparticle sizes than lime-tailings mixture samples with lowerconcentrations of lime additive. The graph of FIG. 5 shows thatincreasing the pH of a tailings stream by adding additional limeadditive to a lime-tailings mixture substantially increases the particlesize of solid particles in the lime-tailings mixture. In one embodiment,the increased particle size may be about 100 microns or larger. Thisincreased particle size can facilitate the dewatering process.

FIG. 6 is a bar graph that shows the measured pH levels of alime-tailings mixture as a function of the concentration of lime in themixture. When the lime-tailings mixture has no lime (e.g., no additivehas been added to the oil sands tailing stream), the pH of the stream isabout 7.9. When the lime-tailings mixture has a lime concentration of1,000 ppm, the pH of the mixture is about 9.2. As the concentration oflime in the mixture continues to increase, the pH of the mixturecontinues to increase, although the rate of change in pH level begins todiminish as the pH level rises with increased lime concentration. Forexample, as shown in FIG. 6, the difference in pH level between alime-tailings mixture with a lime concentration of 5,000 ppm is onlyslightly more than the pH of a mixture with a lime concentration of4,000 ppm and only slightly less than the pH of a mixture with a limeconcentration of 10,000 ppm. As the pH of the mixture approaches 12.3,the pozzolanic reactions between the dissolved calcium cations and theKaolinite occur more readily, causing the dewatering rate to increase,as previously described. This reaction causes calcium silicate hydrate,and aluminum hydrate particles to form on the surface of the Kaolintewhich enhances dewatering.

The far right column of the graph shown in FIG. 6 shows the pH of amixture that has a lime concentration of 4,000 ppm and that has beensubsequently exposed to and treated with CO₂. As previously described,CO₂ that has been added to the mixture will readily react with dissolvedcalcium cations to form insoluble calcium carbonate and water, as shownin reactions 3 and 4 above. When the treatment and dewatering of alime-tailings mixture has finished, dissolved calcium ions from the limeadditive that did not react with the carbonates or clay in the tailingsmay remain dissolved in the water. The presence of the dissolved calciumcations in the water ensures that the pH level of the water remains ator near the elevated level of 12.3, which is too high to safely releaseback into the environment. Over the course of a few weeks, atmosphericCO₂ will react with the dissolved calcium cations and the pH level ofthe release water will decrease. Alternatively, CO₂ may be activelyintroduced to the release water in order to reduce the soluble calciumand the pH level, allowing for recycling this water to the extractionprocess or water release back into the environment. In the example shownin FIG. 6, introducing CO₂ to a lime-tailings mixture that has a limeconcentration of 4,000 ppm reduces the pH of the mixture from 12.2 toabout 8.0, which is almost identical to the pH level of the tailingsstream that had no lime additive.

FIG. 7 is a bar graph that shows the dependency of the yield stress oftailings on (a) the concentration of lime in the lime-tailings mixtureand (b) time. The yield stress is defined as the stress of a material atwhich the material begins to plastically deform, and is representativeof the solids content of the lime-tailings mixture. As shown in FIG. 7,the initial yield stress of oil sands tailings that have no limeadditive added is about 4 Pascal (Pa). After 15 days, the yield stresshas increased to about 15 Pa, and after 1 month, the yield stress isabout 30 Pa. For the lime-tailings mixture that has a lime concentrationof 3,000 ppm, the yield stress is about 4 Pa at the initial stage whenthe dewatering process begins, about 23 Pa after 15 days, and about 180Pa after 1 month. For the lime-tailings mixture that has a limeconcentration of 10,000 ppm, the yield stress of the mixture is about 8Pa at the initial stage, about 640 Pa after 15 days, and about 850 Paafter 1 month. In general, increasing the amount of lime additive addedto oil sands tailings and increasing the length of time that dewateringis allowed to occur will both lead to an increased yield stress of alime-tailings mixture. High doses of lime resulted in higher yieldstress because of pozzolanic reactions that occurred. The pozzolanicreaction produces a weak cementitious material that gradually gainsstrength over time. This chemical modification increases the stresslevel needed to plastically deform reclaimed tailings which isbeneficial for successful reclamation.

FIG. 8 is an image that shows the relationship of the lime-tailingsmixture and the concentration of lime in the lime-tailings mixture.Sample 801 is representative of the starting appearance for each of thesamples. Each of the samples were virtually identical at the start ofdewatering process and each image was taken three weeks after startingthe dewatering process. The only difference between each of the sampleswas the quantity of lime additive mixed in with the oil sands tailings.Sample 802 has 0 ppm of lime additive. As can be seen from the amount ofclear liquid at the top of sample 802, a small amount of solid hasseparated from the water and the clear liquid (e.g., the release water)has accumulated at the top. Samples 803 and 804 have lime additiveconcentrations of 500 ppm and 850 ppm, respectively. Even though limeadditive has been added to these samples, there is little noticeablechange in appearance. This is in part because the concentration of limeadditive has not increased the pH of the system enough to have anappreciable effect on the dewatering and settling rate. Sample 805 has alime additive concentration of 1250 ppm, which corresponds to a pH levelof about 11.0. At this concentration and pH, the amount of clear waterin the sample has significantly increased due to increased settling ofsolid particulate matter in the sample. As previously described,enhanced settling of the solids in the lime-tailings mixture beginsaround a pH of 11.0 in part because there was a noticeable increase inthe amount of soluble calcium in the water, suggesting that calciumcations were reacting with sodium and potassium on the surface of thetailings through cation exchange. As the calcium hydroxide doseincreased beyond 1250 ppm, the settling rate of the mixture was observedto decrease, despite the presence of additional soluble calcium in thewater and larger particle size. It appears that the limited settling athigh lime dose could be related to the interaction of lime with residualorganics in the process water.

In addition to the enhanced settling and dewatering caused by pozzolanicreactions between lime and Kaolinite clay, there may be other reactionsoccurring at elevated pH levels that further enhance the settling anddewatering process of the oil sands tailings. For example, adding limeto a tailings stream may also be useful in the removal of the naphthenicacid from the tailings process water. Naphthenic acids, which aresoluble in the bitumen extraction process water and are often found inoil sands tailings, are toxic and unsuitable for release back into theenvironment. With lime addition up to a pH of 11.0, bitumen particlesrelease from the surface of clay particles in the lime-tailings andfloat to the surface. As more lime is added and the pH level increases,newly-freed calcium cations may react with naphthenic acids in themixture to form calcium naphthenate particles, which is an insolublesolid that is slightly denser than water. The calcium naphthenate solidswill fall to the bottom of the oil tailings mixture along with the restof the dewatered solid material.

Additional tests using four FFT stream samples (e.g., FFT 1-4) were runto corroborate the findings described above. Each FFT sample wasseparated into seven separate test samples and calcium hydroxide wasmixed into each test sample to provide a calcium hydroxide (Ca(OH)₂)concentration of 0 ppm, 1,000 ppm, 2,000 ppm, 3,000 ppm, 4,000 ppm,5,000 ppm, or 10,000 ppm. These concentrations correspond to pH levelsas shown in FIG. 12 (e.g., 1,000 ppm corresponds to a pH of about 9.37).Table 2 (below) shows the original composition of the four FFT streamsbefore Ca(OH)₂ was added, and FIGS. 9-12 show the relationship betweenthe Ca(OH)₂ concentration in a FFT stream and the dewatering ability ofthe FFT stream.

TABLE 2 Fluid fine tailings sample properties Mineral Solids BitumenMethylene Carbonate Content Content Blue Clay Na⁺ Alkalinity FFT % %Index % (ppm) (ppm) pH 1 32.5 2.9 9.2 66 362 579 7.9 2 33.4 1.3 11.2 80877 937 8.3 3 33.8 1.5 9.9 71 827 860 7.6 4 39.6 1.5 8.6 62 894 782 7.7

FIG. 9 is a bar graph that shows the solids content of the separatedcake material after treating the FFT sample with different dosages ofCa(OH)₂ and undergoing pressure filtration. In this test, 150 grams ofFFT-1, FFT-2, FFT-3, and FFT-4 at each dosage were each subjected to 100psi air pressure for up to 120 minutes. A Whatman 50, 2.7 μm filter wasused to separate water from the solids for each FFT sample. The dosagesof Ca(OH)₂ tested for each FFT sample included 0 ppm, 1,000 ppm, 2,000ppm, 3,000 ppm, 4,000 ppm, 5,000 ppm, and 10,000 ppm. Additionally, aninitial measurement was taken from each FFT sample to show the effect ofthe pressure filtration on the solids content of cake. As shown in FIG.9, the average measured solids content of the cake extracted from theinitial sample was about 36%. After being pressurized for up to 120minutes, the solids content of cake extracted from the seven testsamples was measured. The solids content of the cake extracted from the0 ppm Ca(OH)₂ test samples was about 46% on average, indicating that thesolids content of the cake increased by about 10%. For the 1,000 ppm,2,000 ppm, 3,000 ppm, and 4,000 ppm Ca(OH)₂ test samples, the averagesolids content increased along with the Ca(OH)₂. Specifically, theaverage solids contents for the 1,000 ppm dosage corresponded to about47%, the 2,000 ppm dosage corresponded to about 57%, the 3,000 ppmdosage corresponded to about 61%, and the 4,000 ppm dosage correspondedto about 65%. The 5,000 ppm and 10,000 ppm Ca(OH)₂ test samplesindicated a decrease in the solids content of the extracted cakematerial to about 61 and 56%, respectively. Notably, though, the testingof the 5,000 ppm and 10,000 ppm samples was ended prematurely due toissues with the testing apparatus so the reported solids content shownin FIG. 9 for these samples may be underestimated. This overall datasuggests that a concentration of Ca(OH)₂ in FFT samples from about4,000-5,000 ppm may be the ideal concentration to ensure that the solidscontent of extracted cake material is highest.

FIG. 10 is a bar graph that shows the relationship between theconcentration of Ca(OH)₂ in a lime tailings mixture and the rate atwhich release water is separated from an FFT sample using pressurefiltration. The testing conditions that produced the results shown inFIG. 10 were identical to the testing conditions previously describedwith reference to FIG. 9. For test samples with 0 ppm Ca(OH)₂ mixed intothe FFT samples, release water separated from the FFT samples at a rateof about 1.4% of total water present in the test sample per minute. Fortest samples with a 1,000 ppm Ca(OH)₂ concentration, the average releasewater separation rate stayed at about 1.4% of total water per minute.The 2,000 ppm Ca(OH)₂ test samples had an average release waterseparation rate increase to about 2.4%, and the 3,000 ppm and 4,000 ppmCa(OH)₂ samples had average release water separation rates at about 4.0%and 5.2%, respectively. For the 5,000 ppm and 10,000 ppm Ca(OH)₂ testsamples, the average release water separation rate also increased toabout 5.5% and 6.1%, respectively, although the amount of the averagerate increase was less. This overall data suggests that the addition ofCa(OH)₂ (e.g., lime) to tailings streams can be beneficial in increasingthe dewatering rate of tailings.

FIG. 11 is a graph that shows how the number of free calcium cations(Ca²⁺) (e.g., from Ca(OH)₂) in a test sample is dependent on the pH ofthe test sample. As shown in FIG. 11, as pH levels increase up to about11.5, the number of free calcium cations dissolved in the FFT samples'solution decreases. As previously described, this is at least in partbecause the cations react with the sodium and potassium bicarbonates toform calcium carbonate. However, once the pH of the test samples reachesabout 11.5, the number of free calcium cations in the test samplesincrease because the bicarbonates are being depleted. The amount ofcalcium cations continued to increase throughout the test. As the testsample reached a pH of about 12.3, the rate of increase of the calciumcations begins to slowly decrease. As previously described, this couldbe in part due to the pozzolanic reactions, which begin to occur at pHof about 12.0.

FIG. 12 is a graph that shows the relationship between the pH and theconcentration of lime in a lime-tailings mixture for each of the FFT-1,FFT-2, FFT-3, and FFT-4 streams. The relationship between the pH and thecalcium hydroxide dosages shown in FIG. 12 also apply to FIGS. 9-11. Asshown in FIG. 12, a 0 ppm Ca(OH)₂ dosage, the pH level for each of theFFT test samples is about 8.3. At a 2,000 ppm Ca(OH)₂ dosage, there is asteady increase in the pH level of the test samples to about 11.3, afterwhich the rate of change of the pH level decreases. For test sampleswith a Ca(OH)₂ concentration of 3,000 ppm and 4,000 ppm, the average pHlevels are about 12 and 12.2, respectively. Test samples with a Ca(OH)₂of 5,000 ppm and 10,000 ppm maintain a pH level of about 12.3 and about12.4, respectively. This test indicates that increasing the Ca(OH)₂concentration above 4,000 ppm has little effect on the pH of thetailings, suggesting that the maximum acquirable pH level for alime-tailings mixture is less than about 12.5. This finding is congruentwith the results shown in FIG. 9, which shows that a Ca(OH)₂concentration of about 4,000 ppm is the preferred Ca(OH)₂ concentrationto promote enhanced dewatering.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Additionally, aspects of the invention describedin the context of particular embodiments or examples may be combined oreliminated in other embodiments. Although advantages associated withcertain embodiments of the invention have been described in the contextof those embodiments, other embodiments may also exhibit suchadvantages. Additionally, not all embodiments need necessarily exhibitsuch advantages to fall within the scope of the invention. Accordingly,the invention is not limited except as by the appended claims.

Examples of the Present Technology

The subject technology is illustrated, for example, according to variousaspects described below. Various examples of aspects of the subjecttechnology are described as numbered clauses (1, 2, 3, etc.) forconvenience. These are provided as examples and do not limit the subjecttechnology. It is noted that any of the dependent clauses may becombined in any combination, and placed into a respective independentclause, e.g., clause (1, 14, 27, etc.). The other clauses can bepresented in a similar manner.

1. A method for treating oil sands tailings, the method comprising:

-   -   mixing a tailings stream with a lime additive to produce a        lime-tailings mixture having a pH of about 11.0 or greater; and    -   dewatering the lime-tailings mixture to produce a cake including        at least 55% solids by total weight.

2. The method of claim 1 wherein the lime-tailings mixture has a pH ofabout 12.0 or greater.

3. The method of claim 1 wherein the lime tailings mixture has a pH ofabout 12.5.

4. The method of claim 1 wherein the lime additive is a lime slurryincluding one or more of hydrated lime, quicklime, enhanced hydratedlime and dolomitic lime.

5. The method of claim 1 wherein the lime additive includes quicklime,and wherein the quicklime is calcium oxide.

6. The method of claim 1 wherein the lime additive includes hydratedlime, and wherein the hydrated lime is calcium hydroxide.

7. The method of claim 1 wherein the lime additive includes a dolomiticlime including at least 25% magnesium.

8. The method of claim 1 wherein the lime additive includes a limeslurry comprising less than 15% lime by total weight.

9. The method of claim 1 wherein the lime additive includes a limeslurry comprising less than 10% lime by total weight.

10. The method of claim 1 wherein the lime additive includes a limeslurry comprising less than 5% lime by total weight.

11. The method of claim 1 wherein dewatering includes filtering thelime-tailings mixture via pressure or vacuum filtration.

12. The method of claim 1 wherein dewatering includes centrifuging thelime-tailings mixture via a scroll centrifugation unit.

13. The method of claim 1 wherein the cake has a composition thatincludes at least about 60% solids by weight, at least about 65% solidsby weight, at least about 70% solids by weight, at least about 80%solids by weight, at least about 85% solids by weight, or at least about90% solids be weight.

14. The method of claim 1 wherein dewatering the lime-tailings mixtureincludes producing a release water that includes 5% or less solids byweight.

15. The method of claim 14, further comprising recycling the releasewater to be mixed with the tailings stream prior to mixing the tailingsstream with the lime additive.

16. The method of claim 15 wherein recycling the release water increasesthe pH of the tailings stream prior to the tailings stream being mixedwith the lime additive.

17. The method of claim 15 wherein recycling the release water includesreacting calcium ions present in the release water with carbonatespresent in the tailings stream.

18. The method of claim 17 wherein recycling the release water decreasesthe amount of lime additive needed to be mixed with the tailings stream.

19. The method of claim 1 wherein mixing includes mixing the tailingsstream with the lime additive for at least 90-300 seconds.

20. The method of claim 14, further comprising treating the releasewater with carbon dioxide to remove excess calcium.

21. The method of claim 14, further comprising depositing the releasewater into a pond.

22. The method of claim 1 wherein the oil sands tailings comprise wholetailings, thin fluid tailings, thickened tailings, fluid fine tailingsand/or mature fine tailings.

23. The method of claim 1 wherein the oil sands tailings are from atailings pond.

24. The method of claim 1 wherein the oil sands tailings comprise about10-50% solids by total weight.

25. The method of claim 1 wherein mixing does not include using apolymer.

26. The method of claim 1 wherein lime is the only additive mixed withthe tailings stream before dewatering.

27. A method for dewatering oil sands tailings comprising:

-   -   providing a tailings stream including 10-55% solids by total        weight;    -   increasing the pH of the tailings stream by combining the        tailings stream with lime to produce a lime-tailings mixture        having a pH of at least 11.0; and    -   dewatering the lime-tailings mixture to produce a first stream        having 10% or less solids by total weight and a second stream        having 50% or more solids by total weight.

28. The method of claim 27 wherein the first stream is a release waterstream and the second stream is a cake.

29. The method of claim 27 wherein the tailings stream includesparticles having an average first particle size, and wherein increasingthe pH of the tailings stream includes increasing a particle size of theparticles to have an average second particle size larger than theaverage first particle size.

30. The method of claim 29 wherein the average second particle size isabout 25 microns or larger.

31. The method of claim 29 wherein the increased particle sizefacilitates dewatering.

32. A method for treating tailings, the method comprising:

-   -   providing a tailings stream having 10-55% solids by total weight        and a first pH;    -   adding lime to the tailings stream to produce a lime-tailings        mixture having a second pH higher than the first pH; and    -   dewatering the lime-tailings mixture using pozzolanic reactions        to produce a first stream having 10% or less solids by total        weight and a second stream having 50% or more solids by total        weight.

33. The method of claim 32 wherein the tailings includes silicic acid oraluminate, and wherein dewatering the lime-tailings mixture usingpozzolanic reactions include reacting the lime with the silicic acid oraluminate.

34. The method of claim 33 wherein the lime is quicklime and/or hydratedlime.

35. The method of claim 34 wherein reacting quicklime with silicic acidforms calcium silicate hydrate.

36. The method of claim 34 wherein reacting quicklime with aluminatefunctional groups forms calcium aluminate hydrate.

37. The method of claim 32 wherein adding the lime to the tailingsstream induces the formation of insoluble calcium naphthenates.

Additional features and advantages of the subject technology aredescribed below, and in part will be apparent from the description, ormay be learned by practice of the subject technology. The advantages ofthe subject technology will be realized and attained by the structureparticularly pointed out in the written description and claims hereof aswell as the appended drawings.

We claim:
 1. A method for treating oil sands tailings, the methodcomprising: receiving an oil sands tailings stream comprising claybicarbonnates, and 10-45% solids by weight; adding a lime additivehaving a lime concentration of at least 3,000 ppm to the tailings streamto produce a lime-tailings mixture (i) having a pH of about 12.0 orgreater, (ii) substantially free of bicarbonates, and (iii) havingsoluble calcium cations to chemically modify the clay via piozzolanicreactions; and dewatering the lime-tailings mixture to produce a cakeincluding at least 55% solids by weight.
 2. The method of claim 1wherein the lime tailings mixture has a pH of about 12.5.
 3. The methodof claim 1 wherein the lime additive is a lime slurry including one ormore of hydrated lime, quicklime, enhanced hydrated lime and dolomiticlime.
 4. The method of claim 1 wherein the lime additive includescalcium hydroxide and/or calcium oxide.
 5. The method of claim 1 whereindewatering includes filtering the lime-tailings mixture via pressure orvacuum filtration.
 6. The method of claim 1 wherein dewatering includescentrifuging the lime-tailings mixture via a scroll centrifugation unit.7. The method of claim 1 wherein the cake has a composition thatincludes at least about 65% solids by weight.
 8. The method of claim 1wherein dewatering the lime-tailings mixture includes producing arelease water that includes 5% or less solids by weight.
 9. The methodof claim 8 wherein the release water includes lime, the method furthercomprising recycling the release water to be mixed with the tailingsstream prior to adding the lime additive.
 10. The method of claim 9wherein recycling the release water increases the pH of the tailingsstream prior to adding the lime additive.
 11. The method of claim 1wherein adding the lime additive includes mixing the tailings streamwith the lime additive for at least 90-300 seconds.
 12. The method ofclaim 1 wherein the oil sands tailings comprise whole tailings, thinfluid tailings, thickened tailings, fluid fine tailings and/or maturefine tailings from a tailings pond.
 13. The method of claim 1 whereinthe lime additive is a lime slurry including enhanced hydrated limeparticles having a surface area of at least 30 m²/g.
 14. The method ofclaim 1 wherein the tailings stream includes a first pH, and whereinadding the lime additive to the tailings stream is based at least inpart of the first pH.
 15. The method of claim 1 wherein the limeadditive is a lime slurry comprising 10% or less by weight of lime. 16.The method of claim 15 wherein the solids content of the lime slurry isless than 10% by weight.
 17. A method for dewatering oil sands tailingscomprising: providing an oil sands tailings stream including clay,bicarbonates, and 10-55% solids by weight; increasing the pH of thetailings stream by combining the tailings stream with lime to produce alime-tailings mixture having (i) a pH of at least 12.0, (ii)substantially no bicarbonates, and (iii) soluble calcium cations tochemically modify the clay via pozzolanic reactions, the lime having aconcentration of at least 3,000 ppm; and dewatering the lime-tailingsmixture to produce a first stream having 10% or less solids by weightand a second stream having 50% or more solids by weight.
 18. The methodof claim 17 wherein increasing the pH of the tailings stream increases aparticle size of the particles of the lime-tailings mixture.
 19. Themethod of claim 18 wherein an average particle size of the particles isabout 25 microns or larger.
 20. The method of claim 19 wherein thelime-tailings mixture does not include a polymer.
 21. A method fortreating oil sands tailings, the method comprising: providing an oilsands tailings stream having clay, bicarbonates, and 10-55% solids byweight; adding lime having a concentration of at least 3,000 ppm to thetailings stream to produce a lime-tailings mixture having (i) a pH of atleast about 12.0, (ii) substantially zero bicarbonates, and (iii)soluble calcium cations to chemically modify the clay via pozzolanicreactions; and dewatering the lime-tailings mixture to produce a firststream having 10% or less solids by weight and a second stream having50% or more solids by weight.
 22. The method of claim 21 whereinreacting calcium ions with the clay includes reacting the calcium ionswith (a) silicic acid to form calcium silicate hydrate, and/or (b)aluminate to form aluminate hydrate.
 23. The method of claim 21 whereinadding the lime to the tailings stream includes inducing the formationof insoluble calcium naphthenates.